Project Summary/Abstract The androgen receptor (AR)-directed agents enzalutamide and abiraterone acetate are standard first-line therapies for the treatment of patients with metastatic castration-resistant prostate cancer (CRPC). These agents are generally well tolerated and effective at delaying disease progression, prolonging survival, and improving quality of life, but both drugs have limited durations of effectiveness. Patients with refractory disease typically have poor responses to subsequent AR-targeted agents, and there is currently no predictive biomarker that aids in the selection of a second-line therapy. Our group and others have identified upregulation of the glucocorticoid receptor (GR) as one mechanism of resistance to enzalutamide and abiraterone acetate. Our findings suggest that 1) GR may be a biomarker that can predict whether a patient will respond to enzalutamide or abiraterone and 2) blocking GR upregulation in enzalutamide- and abiraterone-resistant patients could result in tumor responses. In preliminary data, we show that isoforms of GR with variable N- termini can be expressed in metastatic CRPC, including one called GR?C that has increased capacity to promote enzalutamide resistance and is detectable in some patient-derived organoid cultures. It will be necessary to characterize GR?C and other GR isoforms in order to develop GR as a biomarker and a therapeutic target. Using a laboratory model of enzalutamide resistance, we have also found that GR expression is epigenetically modulated, and that treatment with an inhibitor of the BET family of proteins suppresses GR expression and restores enzalutamide sensitivity. Expanding on these findings, we will leverage our group's laboratory and clinical expertise to 1) investigate the role of GR isoforms in resistance to enzalutamide, 2) define the broader effects of BET inhibition across a range of preclinical models of prostate cancer, and 3) investigate the activity of a novel BET inhibitor in men with metastatic CRPC whose disease has progressed on enzalutamide or abiraterone acetate.